This invention relates generally to test burners for rockets, and more particularly to a test burner having a flared end vent or nozzle and which is capable of measuring velocity coupled response of solid propellants.
Combustion instability is a phenomenon which occurs readily under a wide variety of conditions throughout rocketry design. This instability can result in a mild vibration, a severe vibration of the rocket, or in extreme conditions, to complete motor failure. The instability usually couples with an acoustic mode of the rocket chamber and exhibits itself as a pressure oscillation. Velocity coupling is usually associated with the axial mode of the combustion chamber.
Basically the burning rate of a solid propellant utilized in rockets depends upon the pressure and gas velocity. When the pressure or gas velocity oscillates in time, the propellant burning rate attempts to accommodate itself to the changing conditions, and this ability is called its response. The response is basically a dimensionless ratio of the fluctuating burning rate to the fluctuating pressure or to the gas velocity. The latter ratio is also known as the velocity coupled response.
Despite the adverse effects velocity coupling instability produces on rockets and missiles, the present state of the art is inadequate to minimize its occurrence. In fact, in many instances this velocity coupling instability is not even considered a factor in rocket design until its presence is detected from firing of the rocket motor. Attempts to alleviate this problem after the fact are usually expensive and ineffective.
Test burners have been designed which have attempted to detect the presence of velocity coupled instability. In such systems presently in use a baseline or reference test is conducted and a propellant sample is then added in order to determine what changes occur because of the sample. The procedure is quite complicated because of two reasons, one is analytical in nature and the other experimental.
Within the prior art test burners, the sample is located to maximize the velocity coupling effect, which however, also places the sample at a location where the pressure coupling and dynamic pressure drop (flow turning) effects are significant. In such a procedure the sample contributions are all additive. The analytical problem of unraveling the velocity coupling in the presence of these other two processes has heretofore never been solved, and, in fact, could not be solved by the prior art, and, therefore a value for flow turning had to be assumed. The experimental problem is that the sample location not only excites the first acoustic mode but also the third mode. It is this latter mode which turns out to interfere with the first mode, the interference mechanism being through amplitude dependent gas dynamic processes. It has therefore been necessary in the past that the pressure amplitude of higher modes be negligible in order to validly measure the velocity response.